I have searched for this question and what I have got from most searches is that:
We can only have 3 "distribution certificates at most" and it is a good practice to use ONLY ONE distribution certificate for all the apps.
However what about the development certificates?
Can I also just use one for all the apps? or do I need individual "development Certificates" for each app?
Yes, it's standard practice to just use one distribution certificate for all the apps.
For development you can use separate Development Certificates.
Development certificates are based on bundle identifiers. Its best practice to use different development certificate for different apps. Still, if you have same bundle id of different apps, you can use same certificate as well.